1. Field of the Invention
This invention relates to an optical disk playing apparatus for automatically discriminating one optical disk to be either of two types of optical disks which have different recording densities for recoding information so as to play the one optical disk, and a method for discriminating the type of a disk.
2. Description of the Related Art
In a disk playing apparatus capable of playing both optical disks, a CD (compact disk) and a DVD (digital versatile disk), which have different recording densities, when an optical disk is set to the playing apparatus, the playing apparatus discriminates the type of the set disk between a CD and a DVD.
A typical method for discriminating an optical disk in the playing apparatus utilizes a fact that the CD and DVD have different pit lengths formed on their surfaces. Specifically, when the disk is rotated at a prescribed rotation speed, the maximum or minimum pit length is measured from the disk, and the type of the disk is discriminated in size of the pit length. In another method, after the maximum or minimum pit length has been detected, the rotation speed of a spindle motor is controlled such that a time length of the pit becomes constant, and the type of the disk is discriminated from the controlled rotation speed. Further, in a known method, the type of the disk is discriminated by utilizing that a PLL (phase locked loop) circuit is locked when a pit with a predetermined length is read out.
In any method for discriminating the type of the disk by utilizing difference of pit lengths as described above, after a disk has been set to the playing apparatus, the disk must be controlled to be rotated to a predetermined rotation speed such as a prescribed rotation speed. Therefore, there is a problem in that a result of the discrimination of the disk type cannot be necessarily obtained before rotation speed of the disk reaches the predetermined rotation speed. Further, there is another problem in that considerably long time is required for discriminating the type of the disk since control of a tracking servo system and a focusing servo system must be performed progressively to a condition where the tracking and focusing errors are within tolerance so that the pits on the disk are read out by a pickup.